This invention is particularly applicable to an apparatus and a method for inductively heating a predetermined portion of a ferrous workpiece and thereafter mechanically deforming the workpiece shape of the heated portion to finish-like tolerances while imparting desired physical properties to such workpiece in an interrupted quench process and will be described with particular reference thereto; however, the invention has broader applications and may be used with any apparatus or process which is capable of selectively heating, in a precisely controlled manner, a predetermined portion of a metal workpiece so as to permit mechanical shaping of the heated portion of the workpiece at a predetermined temperature to impart desired physical properties to such workpiece portion in subsequent surface heat treatments thereof.
The invention will be described with reference to a ferrous workpiece and particularly with respect to plain carbon type steels and alloyed steels which have already been subjected to a case hardened heat treat process. In accordance with broader aspects of the invention however, any metal, ferrous or non-ferrous, which is allotropic or polymorphic may have physical characteristics which can be modified by the low temperature thermomechanical treatment used after selective contour heating of the workpiece.
Isothermal time, temperature transformation curves (hereinafter "isothermal transformation curves") for plain carbon and alloy steels have long been the foundation for heat treating processes for such steels to control their physical properties as well as workpiece distortion, dimensional variation, quench cracking, etc. When the workpiece is a gear, cam, bearing, shaft or the like which is subjected to high contact stresses with other parts, an especially hard, tough, smooth surface must be formed on the case and generally, machine finished within close tolerances. Apart from those heat treat operations which change the chemical composition of the case (such as nitriding), such workpieces are generally produced by infusing or dispersing carbon into the case (at least for low carbon steels) in heat treat operations conventionally known as carburizing, and carbo-nitriding which are followed by cooling the workpiece from an elevated temperature above the A.sub.3 -A.sub.cm temperature to some final temperature at a rate determined by the isothermal transformation curve to achieve certain physical properties. As used hereafter, the phrase "austenitic critical temperature" or "austenitic temperature" will mean that stable temperature whereat austenite or gamma iron will exist as a face-centered cubic structure which for iron and carbon irons is shown as the A.sub.3 -A.sub.cm transformation temperature curve on the iron-carbon phase diagram.
To minimize quench cracks, control distortion and obtain certain desired physical properties related to the grain size of the workpiece, several interrupted quench processes, commonly known as austempering and marquenching have heretofore been used after the workpiece has been carburized. In conjunction with such interrupted quenches, it has also been known for some time that the tensile strength and ultimate strength of high hardenability steels can be significantly increased without loss of ductility by mechanically working or deforming the workpiece in the bainite temperature range of the isothermal transformation curve immediately followed by an oil quench to prevent the formation of non-martensitic transformation products. Such treatment is commonly known as the Ausform Process. More recently, an enhancement in the Ausform Process is disclosed in U.S. Pat. No. 4,373,973 for carburized gears.
In the '973 patent, a gear steel having certain isothermal transformation curve characteristics, after carburizing, is reheated above the austenitic critical temperature and allowed to cool (at a rate sufficient to pass, without contacting, the "knee" of the isothermal transformation curve, (i.e., the "critical cooling rate") to a temperature just above the M.sub.s temperature (i.e., the temperature at which martensite just begins to form and generally shown as the isothermal transformation curve) in a liquid bath whereat the gear teeth are swage-rolled (mechanically worked) while the gear remains in the metastable austenitic condition (i.e., the "ausrolling" process). Ausrolling provides the necessary plastic deformation of the workpiece which importantly must occur before sufficient time has elapsed (in the bay region of the isothermal transformation curve) to allow any phase transformation. Upon completion of the swage-rolling, the gear is then permitted to air cool or is oil quenched to the martensitic range followed by a conventional tempering process. It was found that in gears which were formed from steels having sufficient isothermal transformation curve characteristics to permit ausrolling, there was a fine dispersion of carbides formed during the rolling/swaging operation and that the grain size of the austenite was stabilized to produce a very high dislocation density in the final martensite which was rather uniformly dispersed throughout the surface. Importantly, the grain size and carbon dispersion produced a very smooth surface which could be closely controlled to finish tolerances. As a result, several finish machining type operations now required to manufacture close tolerance gears could be eliminated.
Heretofore, the ausrolling process was limited to those steels having a sufficiently long metastable austenitic range to permit sufficient swage-rolling of the gear to achieve the desired level of plastic deformation. The deformation level must be substantial, typically in a range in excess of 60%, to achieve the desired characteristics. When the workpiece is through heated or even partially through heated by means of conventional standard atmosphere or vacuum furnaces, to a temperature above the austenitic critical temperature, it is difficult to achieve such level of plastic deformation uniformly throughout the surface of the part. That is, if the swage-rolling or mechanical deformation occurs before the workpiece has attained, throughout its core, the desired temperature beneath the knee of the isothermal transformation curve, the metal at the surface of the part where the mechanical working occurs will press against the yielding hotter metal at the core of the workpiece (since the surface will cool before the core does) and deform in a non-uniform manner. If the swage-rolling operation is delayed until homogenization does occur, the time for the plastic deformation operation is reduced (because the phase transformation will occur at a given time) and higher die forming pressures are required with attendant increases in energy costs.